As examples for aryl onium salts usable in the present invention, the ones disclosed in Japanese Patent Laid-opened Nos. Sho 50-151997, Sho 50-158680 and Sho 50-151996 are known, and wherein it is described that these salts can be used as a catalyst capable of curing cationically-polymerizable compounds, such as epoxy compounds, in the presence of any of light, electron beam and radiation, such as X-ray.
As said aryl onium salts have almost no absorbing property for rays having a wavelength longer than 360 nm, which rays are considered as effective range for the curing with ultraviolet radiation, it is known that photocatalytic activity tend to be remarkably deteriorated when such salts are used for photomodeling using lasers having a long wavelength and for photocurable compositions using additives, such as pigments. In particular, such deterioration is seriously disadvantageous in case of photocurable compositions which contain great amount of white pigments, such as titanium dioxide, of which absorption spectrum coincides with the UV absorption spectrum of aryl onium salts. For solving such a problem, investigation was made on use of sensitizers, thereby it is reported that compounds, such as phenothiadine, anthracene and perilene, are useful as such sensitizers for improving photocurable property. However, it is insufficient yet to improve the photocurable property by using said sensitizers mentioned above in case of photocurable compositions using additives, such as pigments.
Also, in case of an aryl iodonium salt represented by diphenyl iodonium salt, of which pair anions are SbF6 and an anion other than B(C6F5), such as PF6, it is known that photocurable property is found to be very bad when such a salt is used for a clear coating composition.
Aiming at solving the problems as described above, it is an objective of the present invention to provide a photocatalytic composition for cationic photopolymerization which can sufficiently cure even aryl iodonium salts represented by diphenyl iodonium salts of which pair aninons contain an anion, such as PF6, and compositions containing additives, such as pigments, within a short time, and is capable of providing excellent physical properties to the products cured by the inventive photocatalytic composition.